Residual strength analysis of ovality and eccentricity and microcrack defects

• 1、School of Mechanical Engineering, Xi'an Shiyou University, Xi'an 710065, China
• 2、Xi'an Key Laboratory of Wellbore Integrity Evaluation, Xi'an 710065, China
• 3、Changqing Downhole Technical Operation Company of CNPC Chuanqing Drilling Engineering Co., Ltd., Xi'an 710021, China

Abstract：Casing is used to support the wellbore of oil and gas wells to form a complete wellbore, and some casings have manufacturing defects such as ovality, inner and outer wall eccentricity and microcracks, which will reduce the strength of the casing. Therefore, it is necessary to analyze the residual strength of the sleeve with manufacturing defects. According to the analysis formula of the anti-extrusion strength of the API casing, considering the difference in the diameter-thickness ratio caused by ovality and eccentricity, the diameter-thickness ratio is corrected accordingly, and the residual strength analysis formula of the defective casing is obtained. Taking the 127mm×7.52mm P110 casing as an example, the residual strength of the casing was analyzed theoretically and numerically, and the residual strength of the casing was compared and analyzed when the ellipticism deformation and the deviation degree of the center of the circle changed in the range of 1%-3%. The analysis shows that when the elliptonality and eccentricity are 3%, the residual strength of the casing is reduced by 6.81% and 7.02%, respectively, and the error is 1.91% and 2.04%, respectively, and the accuracy of the residual strength calculation formula meets the engineering requirements. When the microcracks of the same size are located in the inside, inner and outer walls of the pipe wall, the strength decreases by 8.51%, 3.55% and 4.69%, respectively, and the strength of the microcracks decreases the most when the microcracks are located in the pipe wall, and the strength of the casing decreases approximately linearly with the increase of microcracks. According to the research, a quick reference chart of the residual strength of the casing with different ovality and eccentricity was established, which was convenient for the design and operation personnel. The research results can provide a theoretical basis for the residual strength analysis of 127mm×7.52mm P110 casing with manufacturing defects, and provide a new solution for the problem of cylinders with defects in elastic mechanics.

Keywords： residual strength of casing API formula correction diameter-to-thickness ratio eccentricity crack location